(a) Field of the Invention
The present invention relates to a liquid crystal composition having a high-speed response property and a liquid crystal display using the same, and more particularly to a liquid crystal composition wherein the liquid crystal has a high phase transition temperature, a large birefringence index and modulus of elasticity, and a wide operable nematic temperature range, so that a high-speed response can be obtained to be useful for many liquid crystal devices including LCDs, and a liquid crystal display using the same.
(b) Description of the Related Art
Liquid crystal displays (LCDs) emerged in the early 1970s as display devices for electronic calculators and digital clocks. From word processors and PDAs (personal digital assistants) to notebook PCs, desktop PCs, and TVs, they are now pervading our daily lives. The LCD industry is growing into a huge-scale industry. For a TFT liquid crystal display to be commercially viable in the TV market and to expand its market share, visibility of moving pictures, high brightness, and high-speed response are very important factors.
A liquid crystal is a fluid having optical/dielectric anisotropy. When it is used for a liquid crystal display (LCD), it changes a phase difference of light depending on the voltage applied to a cell, the basic unit of the LCD, due to its optical anisotropy, and thus displays an image. Typical examples of LCD modes are DS (dynamic scattering) mode, TN (twisted nematic) mode, STN (supertwisted nematic) mode, IPS (in-plane switching) mode, OCB (optically compensated bend) mode, and VA (vertical alignment) mode. Currently, the mainstream LCD is a TFT (thin film transistor) LCD, which belongs to the active matrix category.
For a liquid crystal to be used in an LCD, it should have good chemical resistance, photochemical resistance, and heat resistance, and good resistance to electric fields and electromagnetic interference. Also, it should have low viscosity, low threshold voltage, and high contrast, and its operation temperature should be as wide as possible, including below and above room temperature. Additionally, it should have good compatibility in general, since the liquid crystal is mixed with other components. In sum, a liquid crystal having suitable physical properties is required to obtain a high quality LCD. Basically, a liquid crystal requires the following physical properties to be used for an LCD.
First, it should have a wide nematic temperature range. Its melting point should be at least below −20° C. For most nematic liquid crystal mixtures, the nematic phase is maintained even at −40° C., and the phase transition temperature is over 80° C. Recently, as the nematic liquid crystal mixture has been applied to TVs using direct backlights, its phase transition temperature should be over 90° C.
Second, it should have a high specific resistance. Because the liquid crystal functions as a dielectric or a capacitor in the LCD, it should have a high resistance. For a TFT LCD, the specific resistance should be over 1012 Ω cm.
Third, its refractive index anisotropy should be about 0.07 to 0.1, depending on arrangement status of the liquid crystal, operating conditions, contrast ratio, view angle, and so forth of the electro-optical display to be used.
Fourth, the liquid crystal mixture should have a dielectric anisotropy for low-voltage operation, as can be seen from Equation 1. Also, the nematic liquid crystal mixture should have a suitable modulus of elasticity considering the response time. For a liquid crystal mixture used in a notebook PC or a monitor, the threshold voltage is about 1.5 to 2.0 V.
                              V          th                =                  π          ⁢                                    K                                                ɛ                  0                                ⁢                Δ                ⁢                                                                  ⁢                ɛ                                                                        Equation        ⁢                                  ⁢        1            
Wherein Vth is the threshold voltage, Δ∈ is the dielectric anisotropy, and K is the modulus of elasticity.
An LCD used for mobile products should be able to be operated at a low voltage for longer battery use. For the low-voltage operation, a liquid crystal having a high dielectric constant and high phase transition temperature is required.
The TFT liquid crystal display is widely used for telecommunications devices because of its thinness, light weight, and low power consumption. The TFT LCD market is gradually expanding, and they have recently been replacing LCDs in liquid crystal monitors and liquid crystal desktop PCs. Therefore, demand for LCDs is increasing more and more.
Currently, the active matrix liquid crystal display is being spotlighted as a new display capable of replacing CRTs because of its high resolution, high contrast ratio, thinness, and light weight. The liquid crystal displays, which have long been used for electronic calculators, PDAs, and notebook computers, are extending their territory to monitors and liquid crystal TVs. One technical objective to handle ever increasing high-quality, high-capacity display data, and to attain superior moving picture display characteristics, is to develop a liquid crystal composition having a high-speed response property.
Variables of a liquid crystal composition related to the response property of a liquid crystal display have the following relationships.
Equation 1
      τ    on    ∝            γ      ⁢                          ⁢              d        2                            ɛ        0            ⁢      Δ      ⁢                          ⁢              ɛ        ⁡                  (                                    V              2                        -                          V              th              2                                )                    
Equation 2
      τ    off    ∝                    (                  d          π                )            2        ⁢                  γ        1                    K        eff            
Wherein,
γ is the rotational viscosity, d is the cell gap, ∈0 is the dielectric constant, Δ∈ is the dielectric anisotropy (Δ∈=∈81−∈⊥), V is the operating voltage, Vth is the threshold voltage of Frederick's transition, and Keff is the effective elastic constant.
From above equations, it can be understood that the response time can be decreased by reducing the rotational viscosity or increasing the elastic constant of a liquid crystal composition. However, if the rotational viscosity is reduced to improve the response time, the elastic constant and the isotropization temperature (TNI) of the liquid crystal also decrease. And, if the elastic constant is increased, the threshold voltage (Vth) and the rotational viscosity tend to increase. Therefore, a trade-off relationship of the two properties should be minimized to improve the response time.
Most of the currently released or commercially available liquid crystal displays have a response time of longer than 25 ms, which falls short of the time needed to handle moving pictures (about 17 ms, corresponding to one frame). Moreover, the response time should be shorter than 10 ms for practical applications. Although customers don't require such a short response time as yet, it should be attained for the LCD-TV market to grow and for LCDs to win the competition with PDPs and organic EL displays. However, it seems to be very difficult to attain a response time shorter than 10 ms by only improving the liquid crystal properties. Therefore, improvements of the device itself should be accompanied by improvements of liquid crystal properties. In this regard, the most practically accessible approach is to reduce the cell gap of the panel and to develop a liquid crystal appropriate for such a panel. When reducing the cell gap, the refractive index anisotropy of the liquid crystal should be increased considering the view angle and brightness, or the optimal Δnd. In general, if the refractive index anisotropy of a liquid crystal is increased, the rotational viscosity, the elastic constant, and the dielectric anisotropy increase. That is, the factors related to reduction of the response time influence one another. Therefore, development of a new nematic liquid crystal with a lesser trade-off relationship is required.
Growth of the TFT LCD TV market is expected. For the TFT liquid crystal display to have a share in the TV market and to expand it, visibility of moving pictures, high brightness, and high-speed response are very important factors. For high brightness and prevention of liquid crystal deterioration, the phase transition temperature of the liquid crystal should be maintained higher, considering a temperature increase by the tube current of backlights, etc. For high-speed response, the rotational viscosity can be reduced or the cell gap can be decreased to increase the refractive index of the liquid crystal.
The TN (twist nematic) LCD has continuously extended its territory in the display field. It is extending its territory to notebook PCs and TV monitors, and small-to-medium household products. As the LCD-TV market matures and expands, high brightness and high-speed response are emerging as very important factors. For high brightness, the phase transition temperature of the liquid crystal should be maintained higher, considering a temperature increase by the tube current of backlights, etc. TN, IPS (CE), and VA modes are considered as possible TV products. Although the IPS (CE) and VA modes have a wide view angle, the response time is too slow to properly handle moving pictures. Although the TN mode has a narrow view angle, it will be the most competent TV mode in terms of capacity and productivity, if the view angle is improved by a compensation film and if the response time is improved. However, the currently available TN liquid crystal has a low phase transition temperature of about 80° C.
To solve these problems, the following conditions should be satisfied. First, the viscosity of the liquid crystal should be decreased to 20 to 25 mm2/s to improve the response time. Second, the dielectric anisotropy (Δ∈) should be increased to 10 to 15 (35° C., 1 kHz) to reduce the operating voltage. Third, the nematic phase should be maintained in a wide temperature range, preferably in a temperature range of −30 to 80° C. Fourth, the birefringence index (Δn) should be equal to or greater than 0.20 (25° C.).
As described above, a variety of LCD products are being produced in TN, IPS, and VA modes. Most of them use nematic liquid crystals, whose phase transition temperature is about 70 to 80° C., and response time is about 20 to 30 ms.
Accordingly, improvement of response time and increase in phase transition temperature are urgently needed.